The present invention relates to a method for the isolation of olefins from polyolefin plants, and to a method and an apparatus for the preparation of polyolefins.
Gas-phase polymerization is an important technology for the preparation of polyolefins. the catalyst, which is usually employed in supported form, is in the form of small, free-flowing particles which serve as starting points for the polymerization. In general, gas-phase polymerization is carried out as a fluidizedbed process. In this, the catalyst particles are in the form of a fluidized bed during the polymerization. The monomers are generally introduced with the carrier gas into the corresponding polyolefin plant, which is preferably designed as a fluidized-bed polymerization reactor. Catalyst particles are added elsewhere in the polymerization reactor, usually together with inert gas. Particularly suitable catalysts are Philipps (based on chromium/silica gel), Ziegler or metallocene catalysts. The addition of inert gas is necessary since the basic heterogeneously catalyzed polymerization is usually moisture- and/or oxygen-sensitive. The resultant polymer is produced in solid form, which means that the catalyst and polymer exist alongside one another in the fluidized bed. Unreacted monomer is usually circulated thereby dissipating the heat of the reaction.
Gas-phase polymerizations are used, for example, to prepare polyethylene or polypropylene. However, this process can also be used to prepare polymers which are composed of a plurality of different monomer units, so-called copolymers. Suitable catalysts for the gas-phase polymerization are, as described in the introduction, in particular Ziegler, Philipps and metallocene catalysts. Philipps and metallocene catalysts are very sensitive to catalyst poisons, which may suppress the polymerization even in low concentrations (in the ppm range). Catalyst poisons of this type are, for example, sulfur compounds, such as sulfur dioxide or hydrogen sulfide. These frequently occur in such small concentrations that they cannot be detected directly from the gaseous reaction mixture. It is therefore frequently difficult to judge which catalyst poisons are responsible for the inactivity of a catalyst. Since catalyst poisons generally accumulate during continuous operation of gas-phase polymerization plants, it is necessary to remove some of the gaseous reaction mixture from the reactor as offgas during operation.
The disadvantage exists here that olefins are also lost as valuable starting materials if the offgas is not worked up. The offgas in gas-phase polymerizations generally comprises predominantly inert gas and unreacted olefin. If inert gas is not partially removed from the reactor, it accumulates, since it is not reacted in the polymerization. For this reason, it is necessary to continuously remove a constant offgas stream from the reactor in continuous operation. In general, valuable olefin is removed together with inert gas and other secondary components, such as, for example, catalyst poisons, the olefin present in the gas mixture removed is subsequently burnt, and the resultant combustion gas is finally discarded. The loss of olefin is financially significant, and it has consequently been attempted to recover the olefin and to recycle it into the polymerization reactor.
The isolation of the polyolefin from the offgas can be achieved, for example, in accordance with U.S. Pat. No. 5,521,264, by extraction of the olefin, it being necessary to separate the olefin from the extractant in a next step. This separation is quite complex in equipment terms, which means that it is generally more economic and inexpensive not to separate off the olefin and to discard it (after combustion) with the offgas.
The present invention has the object of improving the generic method for the preparation of polyolefins in such a way that the polyolefins present in the offgas can be recovered and subsequently fed back into the polymerization reactor. It is of particular importance here that the recovered olefin is freed both from inert gases and from by-products, in particular catalyst poisons. The equipment complexity should be kept as low as possible here, so that the recovery of the olefins is economically viable.
This object is achieved by a method for the isolation of olefins from a gas mixture comprising one or more olefins, inert gas and catalyst poison, in which the gas mixture is fed to a separation unit and separated into olefin and inert gas. The method according to the invention is then characterized in that an apparatus is connected upstream or downstream of the separation unit and that
i) in the case of an upstream apparatus, the catalyst poison is removed at least partially from the gas mixture, and
ii) in the case of a downstream apparatus, the catalyst poison is removed at least partially from the separated-off olefin.
According to a preferred embodiment of the invention, the gas mixture is produced as offgas in the catalytic gas-phase polymerization of olefins.
The method according to the invention is particularly suitable if metallocene or chromium/silica gel catalysts are used for the polymerization, since these catalysts are particularly sensitive to catalyst poisons. These catalyst poisons are frequently virtually impossible to identify, which means that it is of great importance to remove even extremely small amounts of impurities, even in the ppm range, as completely as possible. In general, the proportion of the catalyst poison in the gas mixture is less than 10xe2x88x923% by weight.
An apparatus which is particularly suitable for removing the catalyst poison in accordance with the invention is one which is designed for mass separation. The term apparatus for mass separation is intended to mean equipment which contains one or more molecular sieves which are able to adsorb, absorb or chemically bind catalyst poison. The equipment can contain, for example, molecular sieve such as activated carbon, zeolites or washing liquid. A suitable corresponding adsorption method is, in particular, pressure sowing adsorption (PSA). However, also suitable in principle are apparatuses with the aid of which catalyst poisons are chemically reacted, with the corresponding secondary products of the catalyst poisons themselves no longer being able to act as catalyst poisonxe2x80x94apparatuses of this type can then release these secondary products again (into the gas stream). A condensation device which operates on the principle of low-temperature separation, is likewise a suitable apparatus, with, in this embodiment, the catalyst poison being removed by fractional condensation.
In the operating variant of the mass separation apparatus by the adsorption method which is particularly preferred according to the invention, the operating temperature in the loading phase is in the range from xe2x88x9230 to 100xc2x0 C., preferably in the range from 10 to 50xc2x0 C., while the pressure in this phase is in the range from 3 to 50 bar, preferably from 10 to 30 bar. In the case of operation with periodic change of the adsorbent or with periodic regeneration of the adsorbent, for example by means of nitrogen, the optimum temperatures for operation of the apparatus are, in accordance with the invention, in the range from 80 to 240xc2x0 C.
Generally the offgas formed in the polymerization process essentially consists (preferably to the extent of greater than 95%) of inert gas and olefin. This gas mixture generally comprises from 20 to 80% by volume, preferably from 40 to 60% by volume, of olefin. The isolation of the olefin from the inert gas is carried out using a separation unit which is, for example, a membrane device which contains one or more membranes, or a distillation device. Membrane devices of this type for the recovery of olefins usually contain one or more diffusion membranes which retain inert gases, such as, for example, nitrogen, and are permeable to relevant olefins (generally to many organic compounds, in particular to numerous hydrocarbons) [M. Jacobs, D. Gottschlich, K. Kaschemekat, Membrane Technology and Research, Inc., Mar. 10, 1998]. The separation into olefin and inert gas is very generally incomplete, meaning both that the separated-off olefin may contain inert gas (residues), and the separated-off inert gas may contain olefin (residues). In this context, the term enrichment of the corresponding components may be used. Suitable separation units are in principle not suitable for the removal or separation of catalyst poisonsxe2x80x94this function is taken on in accordance with the invention by a corresponding apparatus. Thus, the membrane devices employed as separation unit cannot be used for the removal or separation of catalyst poison.
Membranes which are preferred in accordance with the invention are so-called composite membranes, which essentially consist of an impermeable polymer layer, for example of a polydialkylsiloxane, such as polydimethylsiloxane or polyoctylmethylsiloxane, on a porous support material. Suitable support materials are polymers, such as polyesters, polystyrenes, polyamides or also polyolefins. The above-described membranes are employed in accordance with the invention in the form of spiral-wound flat or also cushion modules.
The membrane separation is preferably carried out at temperatures in the range from xe2x88x9230 to 100xc2x0 C., particularly preferably from 0 to 50xc2x0 C., a pressure in the range from 3 to 50 bar, preferably from 10 to 30 bar, advantageously prevailing on the feed side, while the pressure on the permeate side adopts a value of from 0.1 to 10 bar, preferably from 0.8 to 5 bar.
The olefin employed in accordance with the invention is in particular ethylene or propylene. However, it is also possible to polymerize different olefins at the same time, giving copolymers. In principle, all olefins that can be polymerized or copolymerized can be employed in accordance with the invention. Since catalytic gas-phase polymerizations are generally highly oxygen- and/or moisture-sensitive, these are usually carried out in an inert-gas atmosphere. A particularly suitable inert gas is nitrogen. In principle, however, all gases or gas mixtures which have an inerting action in the basic preparation process, for example alkanes, can be employed as inert gas. The catalyst poison separated off is preferably sulfur dioxide.
In addition, an apparatus is provided for carrying out the method according to the invention, this apparatus containing the following devices:
a) a fluidized-bed polymerization reactor,
b) a discharge tank,
c) a filter for the retention of fine polymer particles,
d) an apparatus for the removal of the catalyst poison,
e) a separation unit for the separation of olefin and inert gas,
f) a compressor for recycling the isolated olefin into the fluidized-bed polymerization reactor, and connecting lines between the devices a) to f).